This invention relates in general to an optically generated finger image apparatus and, more particularly, to an apparatus which generates an image from an optical scan of a finger object supported on a platen.
In the art of optical finger image processing, it is desirable to keep the system relatively inexpensive and the size of the system as small as possible while ensuring that the reliability of the system is maintained. In order to get a useful fingerprint image, the contrast between ridge and valley signals must be as great as possible and the background signal must be kept as low and as steady as possible. Further, the apparatus must provide a high resolution image of the fingerprint object.
U.S. Pat. No. 4,322,163 discloses one example of a fingerprint apparatus for generating a fingerprint image to be encoded into machine readable signals. An interrogating slit light beam of substantially collimated light impinges on a finger pressed against a flat platen. The interrogating light beam is at an off normal angle and is displaced across the platen. The finger held on the platen modulates the interrogating light beam to provide a reflected light beam with fingerprint information. The axis of the reflected light beam and the axis of the interrogating light beam are at mirror angles to the normal.
The modulated light beam is imaged onto an array of photo-electric transducers to produce a series of output signals corresponding to the modulated information. The output of the array is serially interrogated at successive scan positions to provide a set of signals containing fingerprint information.
Fingerprint apparatus capable of generating a roll fingerprint image are known in the art. An example having a curved finger retaining platen, is described in U.S. Pat. No. 4,533,837, issued Nov. 19, 1985. That apparatus also employs a collimated interrogating slit light beam at an off normal axis and a modulated reflected light beam at a mirror off normal axis to provide an information signal which is converted to an electrical signal at a photo-electric array.
In addition to the known flat platen and curved platen arrangements there are apparatus which employ a triangular prism as the platen for supporting the finger being interrogated. U.S. Pat. No. 4,210,899 is an example of such an apparatus. Where a prism platen is employed, the interrogating light beam is off normal at an angle of 45 degrees and the axis of the reflected or reading light beam is a mirror image thereof about normal and this is also off normal at 45 degrees.
In all three types of arrangements, five percent (5%) of the incident light is reflected from under the valley/glass interface, obeying the law of reflection, along a mirror axis. About ninety-five percent (95%) of the light passes through the touching surface to be reflectively scattered from the valleys. The ridges of the finger held against the platen effectively squelch geometric reflection because the finger oil on the ridges tends to match the glass index of refraction. Thus, virtually all the light rays headed toward the finger ridges actually contact the ridges and are reflectively scattered back into the glass. The fact that the ridges are substantially in contact with the touching surface and the valleys are not, causes a significant difference in the angular scatter from ridge and valleys. One major difference is that the valley scatter is angularly bounded in space where the ridges are not. This will be later expanded upon.
A second difference is that the five percent (5%) valley/glass interface reflection lies along the mirror axis and the ridge reflection is scattered so that none of the ridge reflection coincides with the same mirror axis. Thus, along the mirror axis, the light is pure valley light and an optical system with an effective aperture approaching zero diameter would only receive valley signals. Such a design would force a system "f" number approaching infinity and thus would have substantially no resolution. Therefore, lens apertures with reasonable dimensions will receive the geometrically reflected five percent (5%) light as well as sufficiently equal ridge and valley scatter to cause a varying signal base line substantially greater than the five percent valley zone light along the mirror axis.
Accordingly, it can be seen that in these fingerprint optical reading systems, the axis of the imaging segment is the mirror image of the axis of the interrogating segment. That is, if the axis of the interrogating light beam is twenty degrees off normal, the axis of the imaging or reading system will also be twenty degrees to the normal in order for the optical aperture of the imaging segment to be centered on the reflected light beam and pick up as much of the 5% of the light that is reflected along the mirror line as possible. Nonetheless, the fact that some 95% of the incident light is reflected in a scattered fashion (as contrasted with along a mirror line) by the valley and ridge zones, means a substantial undesirable background is provided against which it is difficult to pick out the valley signal.
In addition, latent images further obscure the desired image. Finger oil left by previous applications of fingers to the platen provide an image which tends to scatter some of the 5% of light reflected from the valley interface and increase the difficulty of distinguishing the valley signal.
This latent image problem and other related problems were resolved by a technique disclosed in pending U.S. patent application Ser. No. 630,012 filed July 12, 1984, and entitled Direct Finger Reading. The optical fingerprint image generating apparatus disclosed therein does not employ a platen. The finger object is held in position and its surface interrogated by a coherent, collimated light beam. The light beam is preferably shaped in the form of a slit extending along the length of the finger surface being interrogated. The slit beam is scanned around the finger in a rotational motion. The reflected modulated light beam is focused onto a linear array of photo-responsive transducers to provide an electrical signal image of the optical image focused on the array. A limitation on the image so provided is a tendency to lose focus and thus lose image definition and information during a portion of the scan. This out-of-focus problem arises for two reasons. First, the finger being scanned is not a perfect cylinder and, thus, the object position varies along the optical axis. Second, the thickness of the different individual fingers varies greatly so that the extent of the out-of-focus conditions will vary from individual to individual.
Accordingly, an object of this invention is to provide a fingerprint image having high contrast between ridge and valley zones.
Another object of this invention is to provide such an apparatus which avoids swamping out the fingerprint signal with a base line or background signal.
Another object to this invention is to provide this high contrast signal with minimum base line such that the image formed is in focus across the length and width of the finger being interrogated.
A further object of the invention is to provide apparatus which is relatively inexpensive, which is reliable, which provides a repeatable image and which is relatively small in size.
It is an object of this invention to meet the various objects stated above in a device that will be effective with the wide range of finger shapes that exist in the population.